1. Field of the Invention
The present invention relates to the processing of polymer solutions and in particular to process equipment and methodology for preheating polymer/solvent solutions and at least partially devolatilizing the same prior to the final vacuum devolatilization of the molten polymer in short residence time and at low pressure drop.
2. The State of the Prior Art
Preheaters for heating polymer solutions coming from a polymerization reactor prior to vacuum devolatilization are well known in the art. Prior art methodology often features the use of process equipment such as multi tube heat exchanger (MTHE) preheaters, with or without internals (mixing elements). However, such equipment, more often than not, is characterized by flow instabilities due to flashing polymer solution, large shell diameters and thick tube sheets leading to high cost, excessive heating at turndown conditions due to only one fixed heating zone, thermal expansion during burning out processes, and difficult maintenance and cleaning procedures. Other prior art preheating devices include specially designed finned tubes that are mounted directly in the devolatilization chamber. Such a device is described in European Patent Publication no. 0352 727 B1. However, such preheater devices are very expensive and require very large residence times, a process condition which often results in product degradation. Many prior art devices also are characterized by excessive pressure drop.
Furthermore, heat exchangers such as those described in U.S. Pat. No. 4,314,606 (SMR reactor) are known as reactor or coolers for polymers. The standard SMR design has a large liquid hold-up and therefore a large residence time. Due to the typical SMR layout, the service fluid flow must be low in order to keep the service fluid pressure drop in acceptable limits. If the standard SMR design were to ever be used as a polymer devol preheater, the residence time would be typically be 5 to 20 minutes which would reduce the polymer quality.
The present invention provides a polymer devolatilization preheater device and methodology which address the problems encountered during the use of prior art devices as described above. In particular, the invention provides a preheater and methodology whereby the cost of the equipment, pressure drop, residence time, poor temperature distribution and flow instability are all minimized. In accordance with the concepts and principles of the invention, the improved polymer devolatilization preheater comprises an elongated, upright hollow vessel defining a heating chamber having an upper end, a lower end, and a longitudinal axis extending between the ends. The vessel also includes an inlet for a polymer/solvent solution located adjacent to either the upper end or lower end of the chamber and a molten polymer outlet located adjacent to either the lower end or upper end of the chamber. The preheater of the invention further includes at least one heating tube bundle in the chamber.
The bundle may include at least one elongated serpentine heating tube arranged in a configuration having a major axis which extends across the chamber in a direction transverse to the longitudinal axis of the chamber. The tube is positioned such that the polymer/solvent solution descending in the chamber comes into heat transferring contact with an outer surface thereof as the polymer/solvent solution flows along a path that extends from the polymer/solvent solution inlet toward the polymer outlet. In accordance with the broad aspects of the invention, the tube desirably includes a plurality of curved tube portions. The tube also includes a plurality of linking tube portions which interconnect the curved tube portions.
The curved tube portions and/or the linking tube portions may be arranged in a common plane, and such plane may preferably be arranged in essential parallelism relative to the longitudinal axis of the chamber. Ideally, the outer spatial configuration of each tube may be essentially rectangular whereby to present a pair of opposite edges, each of which is disposed in essential parallelism relative to the major axis of the tube configuration.
In a particularly preferred form of the invention, the linking tube portions may be arranged in essential parallelism relative to one another, and the same may be elongated and arranged so as to extend transversely relative to the major axis of the tube configuration.
Desirably, the bundle may include at least two tubes, and such tubes may be arranged in respective adjacent parallel planes. Moreover, the tubes may have respective heating media inlets and outlets which project through a wall of the vessel. The tubes may preferably be arranged such that the heating media inlet of one of the tubes is adjacent the heating media outlet of the other tube, and vice versa. Thus, heating media introduced into the heating media inlets of horizontally adjacent tubes flows through the tubes in opposite directions. Even more desirably, the preheater may include at least two of the bundles in the chamber. One such bundle may be positioned above the other bundle. Ideally, the bundles may be oriented such that the major axes of the elongated serpentine heating tubes of one of the bundles are offset angularly of the longitudinal axis of the chamber relative to the major axes of the elongated serpentine heating tubes of another bundle.
In accordance with one preferred form of the invention, the vessel and thereby the chamber each have a rectangular, preferably square, horizontal cross-sectional configuration. Furthermore, the preheater of the invention may also include an inlet distributor located at the upper end of the chamber for evenly distributing the flow of polymer/solvent solution across an upper portion of an upper bundle and/or an outlet distributor located at the lower end of the chamber. Ideally, the outlet distributor may include a plurality of apertures for dividing the molten polymer into a multiplicity of strands as it leaves the chamber to increase the surface area of the molten polymer and thereby enhance the removal of solvent therefrom in the vacuum devolatilization chamber.
The invention further provides a method for preheating a polymer/solvent solution prior to introduction of the same into a vacuum devolatilization chamber. Such method may include the steps of introducing a polymer/solvent solution into the preheater discussed above through the polymer/solvent solution inlet thereof, heating the solution by allowing the same to descend through the chamber and come into contact with the outer surfaces of the elongated serpentine heating tubes, and recovering a heated, molten plastic at the molten polymer outlet of the preheater chamber. In accordance with another of its aspects, the invention may provide a method for preheating and devolatilizing a polymer/solvent solution which comprises introducing a polymer/solvent solution into the preheater through the polymer/solvent solution inlet thereof, heating the solution and at least devolatilizing the polymer by allowing the solution to descend through the preheater chamber and come into contact with the outer surfaces of the elongated serpentine heating tubes, and directing the heated and at least partially devolatilized polymer through the molten polymer outlet of the preheater and into a vacuum devolatilization chamber. Ideally, the molten polymer may be divided into a multiplicity of individual strands before the same is directed into the devolatilization chamber to increase the surface area of the polymer and enhance the devolatilization operation.